Of the xylene isomers, i.e., ortho-, meta- and para-xylene, para-xylene (PX) is of particular value as a large volume chemical intermediate in a number of applications such as the manufacture of terephthalates, which are intermediates for the manufacture of PET. One source of feedstocks for manufacturing PX is the disproportionation of toluene into xylenes. A disadvantage of this process is that large quantities of benzene are also produced. Another source of feedstocks used to obtain PX involves the isomerization of a feedstream that contains non-equilibrium quantities of mixed ortho- and meta-xylene isomers (OX and MX, respectively) and is lean with respect to PX content. A disadvantage of this process is that the separation of the PX from the other xylene isomers is expensive.
An alternative process for the production of xylenes is by the methylation of toluene using a zeolite or other catalyst composition. For instance, U.S. Pat. No. 3,965,207 discloses the methylation of toluene with methanol using a zeolite catalyst such as a ZSM-5. However, existing toluene methylation processes suffer from the disadvantage that the catalyst tends to deactivate rapidly due to build up of coke and heavy by-products. In addition, the methanol selectivity to para-xylene, the desirable product, has been low, typically in the range of 50 to 60%.
It is also known that alkylaromatic compounds can be synthesized by reacting an aromatic compound, such as toluene, with a mixture of carbon monoxide (CO) and hydrogen (H2) (syngas) at alkylation conditions in the presence of a catalyst system, which comprises (1) a composite of the oxides of zinc, copper, chromium, and/or cadmium; and (2) an aluminosilicate material, either crystalline or amorphous, such as zeolites or clays. See, for example, U.S. Pat. Nos. 4,487,984 and 4,665,238. Such catalyst systems are, however, subject to rapid deactivation and are not capable of producing greater than equilibrium concentrations of para-xylene (PX) in the xylene-fraction product. Typically, the xylene-fraction product contains a mixture of xylene isomers at or near the equilibrium concentration, i.e., 24% PX, 54% MX, and 22% OX. The lack of para-xylene selectivity in alkylation of toluene with syngas can be caused by (1) the presence of acidic sites on the surface of the zeolite, and/or (2) the channel structure of the zeolite not being able to differentiate para-xylene from its isomers.
It is also known that in-situ catalyst-selectivation can increase the selective production of para-xylene. For example, U.S. Pat. No. 5,475,179, which is incorporated herein by reference, teaches a process for selectively producing para-xylene disproportionation of toluene in the presence of a zeolite catalyst that has undergone in-situ silicon selectivation. The selectivation requires adding a silicon selectivating agent to the disproportionation feed and subsequently removing the selectivating agent from the feed when the reaction is switched to a normal production stage. Such processes produce high para-xylene selectivity, but have reduced catalyst activity. Further, the selectivation cannot be performed without switching of the feeds to the reaction. This complicates the selectivation process and requires that certain feeds involved in the selectivation need to be shutdown for the process to be returned to normal production.
U.S. Pat. No. 5,625,103, which is incorporated herein by reference, teaches a continuous toluene disproportionation process which employs both ex-situ silicon selectivation and in-situ coke selectivation. In this process, a silicon-selectivated ZSM-5 catalyst is contacted with a toluene feed for an initial, high temperature adjustment phase, where coke is deposited on the catalyst to enhance its para-selectivity, and for a subsequent steady-state phase, where the toluene is selectively converted to para-xylene.
The present invention seeks to provide a catalyst and process for producing para-xylene by the selective alkylation of benzene and/or toluene with syngas in which a high degree of selectivity can be achieved without diminishing catalyst activity and without addition of separate selectivating agents.